1. Technical Field
This invention relates to a process for catalytic cracking of an oil, particularly to a fluid catalytic cracking (FCC) process which comprises cracking a heavy fraction oil to obtain olefins which are light fraction oils such as ethylene, propylene, butene and pentene.
2. Description of the Prior Art
In a usual catalytic cracking technique, petroleum-derived hydrocarbons are catalytically cracked with a catalyst thereby to obtain gasoline as the main product, a small amount of LPG, a cracked gas oil and the like, and coke deposited on the catalyst is then burnt away with air to recycle the regenerated catalyst for reuse.
In recent years, however, there has been a tendency that a fluid catalytic cracking apparatus is utilized not as an apparatus for producing gasoline but as an apparatus for producing light fraction olefins for use as petrochemical materials. Such utilization of an original fluid catalytic cracking apparatus as an olefin producing apparatus is economically advantageous particularly to an oil refinery in which a petroleum refining factory and a petrochemical factory are highly closely combined.
On the other hand, much attention has been paid to environmental problems, and therefore regulation of the contents of olefins and aromatics in gasoline for automobiles, obligation to add oxygen-containing materials (MTBE or the like), or the like has started to be enforced. In consequence, it can be anticipated that alkylates and MTBE will be increasingly demanded as base materials for high-octane gasoline in place of FCC gasoline and catalytically reformed gasoline. Therefore, it will be necessary to increase the production of propylene and butene which are raw materials for these base materials.
Methods for producing the light fraction olefins by the fluid catalytic cracking of a heavy fraction oil include methods which comprise contacting a raw oil with a catalyst for a shortened time (U.S. Pat. Nos. 4,419,221, 3,074,878 and 5,462,652, and European Patent No. 315,179A), a method which comprises carrying out a cracking reaction at a high temperature (U.S. Pat. No. 4,980,053), and methods which comprise using pentasil type zeolites (U.S. Pat. No. 5,326,465 and Japanese Patent National Publication (Kohyo) No. Hei 7-506389 (506389/95)).
Even these known methods still cannot sufficiently produce light fraction olefins selectively. For example, the high-temperature cracking reaction will result in concurrence of thermal cracking of a heavy fraction oil thereby increasing the yield of dry gases from said oil; the shortened-time contact of a raw oil with a catalyst will be able to decrease a ratio of conversion from light fraction olefins to light fraction paraffins due to its inhibition of a hydrogen transfer reaction, but it will be unable to increase a ratio of conversion of heavy fraction oils to light fraction oils; and, likewise, the use of pentasil type zeolites will only enhance the yield of light fraction oils by excessively cracking the gasoline once produced.